Webster's New World Medical Dictionary

attenuate
To weaken, or to make or become
thin.
attenuated virus
A weakened, less vigorous
virus. An attenuated virus may be used to make a
vaccine that is capable of stimulating an immune
response and creating immunity, but not of causing
illness.
atypical
Unusual, or not fitting a single diagnostic 
category.
atypical measles syndrome
The modified
expression of measles, as may occur in persons who
were incompletely immunized against measles or
who have compromised immune systems.
Abbreviated AMS. AMS begins suddenly with high
fever, headache, cough, and abdominal pain. A rash
may appear 1 to 2 days later, often beginning on
the limbs. Swelling (edema) of the hands and feet
may occur. Pneumonia is common. See also
measles.
audiogram
A test of hearing at a range of sound
frequencies.
audiology
The study of hearing.
audiometry
The measurement of hearing.
auditory acuity
The clarity or clearness of hear-
ing, a measure of how well a person hears. Auditory
acuity is measured in order to determine a person’s
need for a hearing aid.
auditory tube
See 
Eustachian tube.
augmentative communication device
A physi-
cal, mechanical, or electronic device that helps a
person with a speech impairment to communicate.
Augmentative communication devices range from
books of pictures or words that the patient can
show to express thoughts, to computers that are
capable of synthesizing complex speech.
aura
A sensation perceived by a patient that pre-
cedes a condition affecting the brain. An aura often
occurs before a migraine or seizure. It may consist
of flashing lights, a gleam of light, blurred vision, an
odor, the feeling of a breeze, numbness, weakness,
or difficulty in speaking..